create doc structure for the aspera add-on

the aspera add on will now have releases independent of the platform and other nuxeo server content. as such, we need a documentation structure that allows us tot include things such as release notes and update instructions. Could we get 2 sub pages to aspera docuementation (https://doc.nuxeo.com/nxdoc/nuxeo-aspera-connector/) please: 'release notes' and 'upgrade notes'.